1. Field of the Invention
The present invention relates to a system and a method for processing communication signals to more efficiently achieve channel estimation, particularly in providing channel estimation in an orthogonal frequency division multiplexing (OFDM) receiver that performs frequency domain processing.
2. Description of the Related Art
To increase data rates and mitigate multipath, advanced networks including so-called 4G wireless networks such as WiMAX and LTE have adopted variations of the orthogonal frequency division multiplexing (OFDM) waveform for their PHY layer. The PHY layer is the physical, electromagnetic means by which bits of information are transmitted and received over the air or wire. OFDM offers much sought-after bandwidth efficiency, with a built-in mitigation for the multipath of the wireless channels in urban environments. The sensitivities of OFDM transmission are well-understood. The “bit-pump” scheme for the PHY layer has proven successful in digital subscriber line (DSL, wired) OFDM applications. On the other hand, mobile wireless OFDM applications still face challenges to achieve OFDM's designed capacity.
At the core of the practical and theoretical advantages of OFDM is the use of a fast Fourier transform (FFT). The FFT implemented in OFDM can be viewed as analogous to a bank of tuners for Nc-simultaneous radio stations because each of the tones generated by the FFT can be independently assigned to users. The OFDM PHY provides or receives a simultaneous blast, over a short period of time, of bits on each subcarrier frequency (tone) with a complete, or partial, allocation of subcarriers to a given user. Making a partial allocation of subcarriers among different users and aggregating many users within one period is one multiple-access scheme for OFDM. In the case of 10 MHz bandwidth channels, a user can be receiving up to Nc=840 (WiMAX) or 600 (LTE) simultaneous tones, over a very short duration, such as 0.1 milliseconds. These Nc-tones per period of time make up an OFDM symbol. The allocation of many users in one symbol is called OFDMA.
Wireless standards usually consist of three important time segments, defined by the bandwidth available and the information's time sensitivity. A number of symbols are concatenated to define a frame, which is the longest relevant unit of time and for example might be ten milliseconds. If the standards assign twenty symbols to a frame, then the symbol duration is 0.5 milliseconds. Finally, the FFT size and cycle prefix duration define the time spacing between samples, so a 1024 point FFT and 128 point CP define a sampling time of 43 nanoseconds. Although FFT computations can be comparatively efficient, the FFT size for an exemplary OFDM system is sufficiently large (e.g., 1024 samples in the 10 MHz bandwidth case) that computational demands remain rather high and power consumption remains an important constraint in designing receivers for user handsets.
OFDM systems are more sensitive and have less robust signal acquisition than 3G systems based on code division multiple access (CDMA). The sensitivity of OFDM systems comes from their use of the fast Fourier transform (FFT) to transform incoming signals from the time to frequency domain. The FFT in OFDM systems can deviate from ideal assumptions under very common real-world conditions and receiver implementations. If the assumptions underlying the FFT algorithm fail, cross talk develops between all of the Nc-channels (on Nc subcarriers) being transmitted. Crosstalk between subcarriers degrades performance, which in turn causes bit error rates (BER) to increase.
A wireless OFDM handset may receive multiple paths (copies with different delays) of the same signal from a transmission tower (“base station”) due to reflections from structures or large water surfaces. This non-line-of-sight reception or multipath causes the signal to be distorted from the flat frequency domain “shape” output by the transmitter. A receiver must compute a filter to restore the signal to its original flat spectral shape; that filter is said to equalize the signal. OFDM receivers perform a critical equalization computation for each OFDM symbol transmitted.
OFDM, unlike most other modulation strategies commonly used in communication systems, can include two equalizers to improve signal quality: a time equalizer (TEQ) and a frequency equalizer (FEQ). Some OFDM applications such as DSL include a time equalizer while others, such as systems that implement current wireless standards, do not demand a time equalizer. All practical OFDM receivers have a frequency equalizer. Whether a receiver includes a time equalizer or only a frequency equalizer, the receiver needs to perform channel estimation to at least initially determine values of the equalizer coefficients before the equalizer can be used to improve the signal quality. Determining the coefficients for frequency equalizers typically is performed in the frequency domain.
FIG. 1 schematically illustrates an OFDM communication system, including an OFDM transmitter 10 that generates a radio signal modulated with information such as data generated by a computer network or voice data. The radio signal travels over the channel 12 to a receiver 14. Channel 12 distorts the radio signal in various ways, including by transmission over multiple paths of different lengths, introducing multiple copies of the radio signal with different offsets and amplitudes in the mechanism known as multipath. Conventional OFDM receiver circuitry 14 down converts the received signal to baseband and then analog-to-digital converts that signal to produce the information signal that is input into the OFDM processing circuitry shown in FIG. 1. The radio signal is input to an alignment element 16 that aligns the signal temporally so that it can be processed according to transmission standards. Following the alignment element 16, the signal is passed to a processing element 18 that removes the cycle prefix (CP) from the signal. A conventional OFDM transmitter 10 adds a CP of length NCP, which consists of the last NCP samples, to a unique signal waveform of length N so that the digital signal that the transmitter converts to analog and transmits is of length N+NCP. An initial step of the receiver's reverse conversion process then is to remove and discard the added NCP cycle prefix samples. Following that step, a serial to parallel conversion element organizes and converts the serial signal into a parallel signal for further processing. The cycle prefix can be removed either before or after the serial to parallel conversion.
After CP removal 18 the parallel data is provided to a fast Fourier transform (FFT) processor 20 that converts the time domain samples s(n) to a set of frequency domain samples Rj(k) for processing. The received OFDM symbol is assumed to be corrupted by the channel, which is assumed for OFDM to introduce amplitude and phase distortion to the samples from each of the subcarrier frequencies used in the OFDM system. The FEQ 22 applies an amplitude and phase correction specific to each of the frequencies used in the OFDM system to the various samples transmitted on the different frequencies. To determine the correction to be applied by the FEQ 22, the FEQ 22 needs an estimate of the channel's amplitude and phase variations from ideal at each frequency.
A conventional OFDM channel estimator 24 used in FIG. 1 typically receives and estimates a channel based on a set of pilot tone locations 26 or another signal that has predictable characteristics such as known bits and subcarrier locations. This is termed frequency domain channel estimation or FDCE. The pilot tones are generally dictated by the relevant standards. It may be necessary to interpolate from the received information to provide channel estimate information for each subcarrier or tone. All FDCE implementations react to the FFT output OFDM symbol to extract the pilots. The channel estimate at each pilot may be determined as the amplitude and phase rotation from the ideally expected post-demodulation value of “+1” for each pilot. That is, any deviation from this “+1” value constitutes the distortion from the channel at that frequency's bandwidth. The value of the channel at the data subcarrier frequencies can be estimated by interpolating the values obtained at the pilot subcarrier frequencies. Various improvements on simple channel estimation schemes are known and are conventionally implemented in the frequency domain. The frequency equalizer 22 receives the signals from the fast Fourier transform processor 20 and the channel estimates from the estimator 24 and equalizes the signal. The output of the equalizer 22 typically is provided to a parallel to serial element that converts the parallel outputs of the equalizer to a serial output user signal.
An OFDM symbol is constructed by setting active data subcarrier values to non-zero values from a prescribed set of values according to the number of bits to be “loaded” into that OFDM symbol. These values are then subjected to an inverse fast Fourier transform (IFFT) to obtain the time-domain samples. Then, a cycle prefix is appended to the beginning of the symbol by taking a defined number of samples from the end of the symbol's time-domain samples. If the IFFT produces 1024 samples, then the number of time-samples is 1024. Certain standards select the CP to have length 128. That means the transmitter selects the last 128 samples from the sequence of 1024 samples and pre-pends those samples so that they become the first 128 samples in the transmitted OFDM symbol, which has a total of 1152 samples. Because of this construction, selecting any 1024 samples out of the 1152 samples of the OFDM symbol produces a circular shift on the original 1024 OFDM time samples.
In the case of the WiMAX standard, the OFDM symbol can be transmitted on 60 subchannels with 14 active subcarriers per subchannel, for a total of 840 active subcarriers, with 4 pilots per subchannel. The location of the pilots in any given symbol, and therefore subchannel, is prescribed by the standard.
One theoretical advantage of OFDM is that equalization can be performed after the FFT for each received tone individually and through a rather simple algorithm. Another advantage that enables OFDM receivers is that equalizer coefficients need only be estimated for each subcarrier that is relevant to the user, a quantity smaller than the FFT size. The values for each equalizer coefficient corresponding to each tone will depend on the estimation of the channel coefficient—termed channel estimation. Like many operations in OFDM receivers, typical OFDM receivers perform channel estimation after the FFT because the channel estimation at that point is performed simply and efficiently based on a user's tone allocation. Because channel estimation is performed after the FFT, the tones will be impacted by FFT and post-FFT distortions, known as inter-carrier interference (ICI). ICI generally manifest through three conditions: 1) errors in frequency tuning; 2) doppler from mobility; and 3) interference from other cell-sites. OFDM systems accommodate inter-symbol interference by providing a time gap between symbols, so that inter-symbol interference generally is of less concern for OFDM as compared to other wireless schemes.
Any given channel has a well-known limit to its capacity. In current OFDM implementations, there are additional losses in capacity below the expected rates. Channel estimation errors are a principal culprit. Since ICI affects the channel estimation algorithms post-FFT in typical implementations, poor channel estimation leads to inaccurate equalizer coefficients. Increased bit error rate (BER), due to myriad conditions such as demanding channels and poor channel estimation, can be accommodated by reducing the transmitted bit rate offered to a user. In effect, reducing the transmitted bit rate allows for robustness against interference. However, this is a non-linear correction, since the OFDM scheme allows for transmission of two, four or six bits per tone and consequently, under some circumstances, mitigating distortion requires fewer than 2 bits/tone be transmitted, which means the system makes no data available to the user at all.